1. Technical Field
The present invention relates to equipment used in the fabrication of semiconductor devices. More particularly, the present invention relates to a method and apparatus for normalizing a light beam to a wafer surface in semiconductor fabrication equipment.
2. Description of the Prior Art
The process of etching a pattern in a semiconductor wafer becomes increasingly critical as device geometries shrink. Control over the rate and depth of etch is absolutely essential to the fabrication of modern integrated circuits. Techniques used to control etching rely on detection of the process end point, i.e. the point at which the etch cycle is to be concluded, taking into account depth of etch, time necessary to effect etchant flow rate reduction and chamber purge cycles, among other criteria.
An end point detection system, etch rate monitor, or etch-to-depth controller of the type currently in use is shown in FIG. 1. Such system involves a coherent, collimated light source, such as a laser beam source 12, which is typically a Helium-Neon (HeNe) laser, a Helium-Cadmium (HeCd) laser, or other such light source. The laser beam is projected through an optical train 30, including a beam expander (as with a HeNe laser), to a wafer surface 18. The beam is on the order of 6 mm wide such that a large portion of the die is covered. Depending upon the geometry of the semiconductor dies, the width of the beam may be reduced or increased. Since portions of the surface of a silicon wafer are highly reflective, the zero-order reflections are reflected back toward the source, if the beam is at normal incidence.
As the depth of the various patterns etched into the wafer surface increases, portions of the laser beam are scattered, creating a diffraction or interference pattern in the reflected laser beam which is separately detected by a collecting and detector apparatus 11 and computer processed to indicate rate and depth of etch, and from which signals may be derived to control the end point of the etch process.
It is essential that a coherent, collimated light source be normal to (i.e. at 90.degree. to) the wafer surface in a diffraction laser etch rate monitor of the type described above, especially when devices are to be fabricated using submicron geometries. It is known in the art to perform such normalization by manually entering a slip of paper into the fringes of the illumination laser beam to determine if the zero order reflected beam coincides with the illumination beam. The angle of incidence is then corrected to normal by repositioning the laser source's angular relationship to the wafer.
Another known technique for effecting laser beam normalization is to position a beam splitter in the laser illumination beam path and arrange a target at one illumination path of the beam splitter, such that coincidence of the source and zero-order return beams may be observed on the target. Adjustments to correct for an angle of incidence that is normal may then be made as outlined above.
Such techniques as are known for achieving a normal angle of incidence between a laser beam and a reflective wafer surface pose particular problems. Using a piece of paper exposes a technician performing the adjustment to a dangerous laser beam and may result in serious injury to the technician's eyes. Since the paper is hand held, the adjustment is at best very crude and inadequate for submicron device structures. More importantly, operating technicians are not generally familiar with the construction of the etch rate monitor, nor do they understand the phenomena involved, although they can be readily instructed in performing simple calibration steps.
Using a beam splitter and target produces more accurate results than the typical adjustment procedures using a strip of paper, but attenuates the power of the laser beam. Thus, more powerful and expensive laser sources must be designed into the etch monitor equipment. The higher power levels increase the potential for injury to a technician and may also damage delicate semiconductor device structures. Also, beam splitters alter beam polarization properties and cause unwanted beam distortion.